Position sensor

ABSTRACT

A position sensor for detecting a position by comparing a detected level with a threshold level. This operation is performed without being affected by the characteristics intrinsic to the used sensor means or by power supply variations. Furthermore, the operation is not varied even if the sensor means ages. The sensor means is caused to shift from a first operating state to an unoperated state and then to a second operating state according to operation of an object whose position should be detected. The level of the output from the sensor means is compared with a threshold level. Thus, the timing at which the sensor means shifts from the unoperated state to the second operating state is detected. The output level detected when the sensor means is in the first state is stored in a capacitor and divided by resistors to obtain the threshold level. Therefore, if the characteristics of the used sensor means deviate from ideal characteristics, or if power source variations occur, then the threshold level is automatically corrected. Consequently, the operation of the sensor means is not affected by these deviation and power source variations.

TECHNICAL FIELD

The present invention relates to a position sensor and, moreparticularly, to a position sensor best suited for detection of aposition in an apparatus (e.g., a device for detecting the position of ashutter blade in a still camera) which is equipped with a power supplyproducing an unstable output voltage level such as a dry battery or drycell and in an apparatus which is not expected to be periodicallyinspected by the user.

BACKGROUND ART

A known position sensor compares the output of a sensor means with agiven threshold level to detect the position of an object whose positionshould be detected, to control the position of the object, and toperform a timing control operation. We now describe an apparatus fordetecting the position of a shutter blade included in a shuttermechanism as an example. Generally, immediately after the opening formedby shutter blades is released, the operating characteristics of theshutter blades are not stable by the effects of the camera's posture andfor other causes. In order to obtain an adequate exposure time, acertain preparatory interval is required until the operatingcharacteristics of the shutter blades become stable. Accordingly, in anordinary shutter mechanism, after the opening formed by the shutterblades is released, the blades go through the preparatory interval andmove until a pinhole is formed. This state is detected. In response tothis, the exposure time and the timing at which the strobe issynchronized are controlled.

Good examples of this position sensor are widespread position sensorsusing a light switch such as a photo-interrupter or photo-reflector. Insuch a widespread position sensor, shutter blades are moved so as toblock and unblock an optical path going from a light-emitting portion toa light-receiving portion. FIG. 6 illustrates the principle of oneexample of a blade position detector using a photo-reflector. In theillustrated circuit, a reflector 3 is disposed in an optical path goingfrom a light-emitting diode 1 to a phototransistor 2. The front end, forexample, of a shutter blade 4 is made to pass through the optical pathcontaining the reflector 3. The shutter blade 4 blocks the optical pathand then releases it. At this instant, the phototransistor 2 is turnedon. This turns on a switching transistor 5. At this instant, an exposurecontrol circuit 6 is started to be triggered.

However, the output from the light-emitting diode, the reflectivity ofthe photo-reflector, the sensitivity of the phototransistor, and otherfactors vary among different commercial products. Therefore, where theabove-described method is employed, one conventional technique is toadjust the output from the light-emitting diode 1 by means of a variableresistor 7 or the like installed in a circuit when the apparatus isshipped. However, the camera user is not urged to periodically inspectthe camera with certainty. Consequently, this method cannot cope with asituation in which the camera ages and the performance of each componentof the photo-reflector changes. Another problem with a camera is thatits power source depends on a dry battery or dry cell. That is, thestability of the power source is quite low. Hence, an accuratevoltage-regulated circuit is necessitated.

DISCLOSURE OF THE INVENTION

In view of the foregoing problems, the present invention has been made.It is an object of the invention to provide a position sensor which isnot adversely affected if components such as a photo-reflector age andwhich does not require an accurate voltage-regulated circuit.

The present invention is based on a position sensor comprising a sensormeans having a detecting portion and a comparator means for comparingthe output level from the sensor means with a threshold level and forproducing a signal indicating the position. The detecting portion isdisposed along the operating line of an object whose position is to bedetected.

In one feature of the invention, the output level is preliminarily readprior to the operation for detecting the position of the object when itis operating. The threshold level of the comparator means is establishedin response to the read output level.

In the novel structure constructed above, before an actual operation fordetecting the position, the output level from the detecting means ispreliminarily read. In response to the output level from the sensormeans at the instant of this reading, the threshold level of thecomparator means is established. Therefore, it is not necessary toadjust the operating point according to the characteristics of the usedsensor means or to install a voltage-regulated power circuit. Also, ifthe sensor means ages and deteriorates in performance, the thresholdlevel can be automatically corrected each time a photographic operationis performed.

Preferably, the novel apparatus is equipped with a sensor means whoseanalog output varies, depending on whether the detecting portion isoperating or not. The object whose position is to be detected has aportion that causes the detecting portion to make a transition from afirst operating state to an unoperated state and then to a secondoperating state when the object is operating. When the detecting portionis in the first operating state, the threshold level of the comparatormeans is established in response to the output from the sensor means. Inthis way, the threshold level can be automatically corrected in the sameway as in the above-described structure.

More preferably, in the second operating state, the object is activatedin second operating conditions. This can bring the load conditionsimposed during the preliminary reading into agreement with the loadconditions imposed during actual reading. In consequence, the effects ofvariations in the load on the sensor can be circumvented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a position sensor according to theinvention;

FIG. 2 is a circuit diagram of an analog circuit embodying the conceptof the invention;

FIG. 3 is a block diagram illustrating software processing for embodyingthe invention;

FIG. 4 is a flowchart illustrating the example shown in FIG. 3;

FIG. 5 is a perspective view of another example of the invention; and

FIG. 6 is a circuit diagram of the prior art position sensor.

BEST MODE FOR CARRYING OUT THE INVENTION

A shutter mechanism acting also as a stop is described in detail byreferring to the drawings. This mechanism is one example of theinvention. FIG. 1 shows a shutter mechanism to which the invention isapplied. A shutter blade 10 is swingably held to a shaft 12 on the baseplate 11. Another shaft 13 is mounted on the rear surface of the baseplate 11. An opening lever 14 is swingably held to the shaft 13. Anengaging pin 14a is attached to the front end of the lever 14, extendsthrough the base plate 11, and is engaged in a slot 10a formed in theshutter blade 10. Therefore, when the opening lever 14 is rotated to theright about the shaft 13, the shaft blade 10 turns to the right aboutthe shaft 12 while the slot 10a is engaged by the engaging pin 14a. As aresult, the blade 10 opens an aperture 11a formed in the center of thebase plate 11.

A photo-reflector 15 comprises a light-emitting diode 15a and aphototransistor 15b which are a light-emitting portion and alight-receiving portion, respectively. This photo-reflector 15 ismounted in a manner not illustrated but the light-emitting surface ofthe light-emitting diode 15a and the light-receiving surface of thephototransistor 15b are mounted in front of the shutter blade 10 so asto face the base plate 11. A reflector 11b having a high reflectivity isformed by evaporation or other method at the location on the base plate11 at which the light-emitting diode 15a emits light. If thereflectivity of the base plate 11 itself is high, the base plate 11itself can be used as a reflector without performing evaporation or thelike.

A blocking blade piece 10b is formed integrally with the front end ofthe shutter blade 10 to block and unblock the optical path going fromthe light-emitting diode 15a to the phototransistor 15b. In the presentexample, the blocking blade piece 10b unblocks the optical pathextending from the light-emitting diode 15a to the phototransistor 15bunder the initial condition. The blade 10 rotates to the right about theshaft 12 so as to open the aperture. This blocks the optical path goingfrom the light-emitting diode 15a to the phototransistor 15b. The bladefurther opens the aperture and unblocks the optical path extending fromthe light-emitting diode 15a to the phototransistor 15b immediatelybefore the shutter blade 10 forms a pinhole. In the present example,when the shutter blade is in its initial position, the light-emittingdiode 15a is made to emit, which is hereinafter referred to as theprevious emission. The present example is characterized in that thethreshold level is established in response to the output from thephototransistor 15b at the time of the previous emission. In the figure,only the shutter blade 10 is shown. Another shutter blade whose shapehas an essentially symmetrical relation to the shape of the shutterblade 10 is pivoted to a shaft 16. This shutter blade (not shown) isrotated to the left about the shaft 16 when the opening lever 14 rotatesto the right, and opens the aperture 11a.

FIG. 2 is a circuit diagram of a first example of the invention.Indicated by 10b, 11b, 15a, and 15b are the aforementioned blockingblade piece 10b, reflector 11b, light-emitting diode 15a, andphototransistor 15b, respectively. The photocurrent produced from thephototransistor 15b is varied by the intensity of light incident on thephototransistor 15b, the light being emitted by the light-emitting diode15a. A voltage developed across a resistor 20 is varied by thephotocurrent, and this voltage is applied to the noninverting input of acomparator 21. The present example is characterized in that a capacitor22 is electrically charged according to the photocurrent produced fromthe phototransistor 15b during the previous emission, and that thethreshold level applied to the inverting input of the comparator 21 iscontrolled according to the charging level of the capacitor 22.

More specifically, the output from a voltage follower 23 having acontrol input is applied to the terminal point of the capacitor 22. Whena control signal S1 applied from a CPU 24 goes high, the voltagefollower 23 is enabled, in which case the level developed across theresistor 20 is applied to the capacitor 22. Then, the capacitor 22 ischarged to the terminal level of the resistor 23.

The charging level of this capacitor 22 is applied via a voltagefollower 25 to a series combination of resistors 26 and 27. The voltagelevel is approximately halved by the resistors 26 and 27 and applied tothe inverting input of the comparator 21. A switching transistor 28 fordischarging the capacitor 22 is caused to conduct when a control signalS2 applied from the CPU 24 goes high, thus releasing the electric chargestored in the capacitor 22.

The operation of the present example is described next by referring tothe items described above. First, in the initial state, the shuttermechanism assumes the state shown in FIG. 1. The aperture 11a is blockedby the shutter blade 10 and the other shutter blade (not shown) pivotedto the shaft 16. The blocking blade piece 10b unblocks the optical pathgoing from the light-emitting diode 15a to the phototransistor 15b.

Under this condition, driving pulses are supplied to a stepper motor(not shown). When the stepper motor rotates a given number of steps, adisengaging member (not shown) releases the opening lever 14. The lever14 rotates to the right about the shaft 13. The present example ischaracterized in that the light-emitting diode 15a is made to emitpreviously after the supply of the driving pulses to the stepper motoris started and before the opening lever 14 is released.

More specifically, after the driving pulses are supplied to the steppermotor, a driving current is supplied to the light-emitting diode 15abefore the opening lever 14 is released. The control signal S1 is madeto go high, while the control signal S2 is caused to go low.

During this previous emission, the blocking blade piece 10b unblocks theoptical path going from the light-emitting diode 15a to thephototransistor 15b. Therefore, light emitted by the diode 15a isreflected by the reflector 11b and enters the phototransistor 15b. Avoltage corresponding to the current flowing via the phototransistor 15bis developed across the terminals of the resistor 20.

At this time, the voltage follower 23 is enabled by the control signalS1 which is in the high level and so the voltage follower 23 deliversthe voltage developed across the terminals of the resistor 20. Thetransistor 28 is cut off by the control signal S2 that is in the lowlevel. Consequently, the capacitor 22 is charged to the output level ofthe voltage follower 23 corresponding to the terminal level at theresistor 20. The terminal level across the capacitor 22 is applied viathe voltage follower 25 to the series combination of the resistors 26and 27. The voltage level approximately halved by the resistors 26 and27 is applied as the threshold level to the noninverting input of thecomparator 21.

Subsequently, if the opening lever 14 is released, the shutter blade 10rotates to the right about the shaft 12 while the pin 14a is engaged inthe slot 20a. During this clockwise rotation of the shutter blade 10,the blocking blade piece 10b blocks the optical path going from thelight-emitting diode 15a to the phototransistor 15b before the shutterblade 10 forms a pinhole in practice. Immediately before the blade 10forms the pinhole actually, the blocking blade piece 10b again unblocksthe optical path going from the light-emitting diode 15a to thephototransistor 15b.

During the clockwise rotation of the shutter blade 10, if the blockingblade piece 10b blocks the optical path going from the light-emittingdiode 15a to the phototransistor 15b, then a photocurrent no longerflows from the phototransistor 15b to the resistor 20. The voltageacross the terminals of the resistor 20 substantially drops to groundlevel. The level at the noninverting input becomes sufficiently lowerthan the level at the inverting input of the comparator 21. Inconsequence, the output from the comparator 21 goes low.

Subsequently, if the shutter blade 10 rotates further to the right, andif the blocking blade piece 10b again unblocks the optical path goingfrom the light-emitting diode 15a to the phototransistor 15b, then aphotocurrent flows from the phototransistor 15b to the resistor 20. Thisphotocurrent causes the voltage across the resistor 20 to be applied tothe noninverting input of the comparator 21. Meanwhile, the level at theinverting input of the comparator 21 is obtained by dividing thecharging level of the capacitor 22 approximately by a factor of two bymeans of the resistors 26 and 27. As described already, the charginglevel of this capacitor 22 is equal to the voltage across the resistor20 during the previous emission, the voltage being stored. In anadditional feature of the present example, the previous emissionoperation is performed while supplying the driving pulses to the steppermotor. Therefore, the conditions under which the phototransistor 15boperates during the previous emission substantially agrees with theconditions under which the phototransistor operates during actualdetection operation. It is assured that the level at the noninvertinginput of the comparator 21 is higher than the threshold level which isobtained by dividing the charging level of the capacitor 22 by a factorof approximately 2. The output from the comparator 21 is inverted, i.e.,changes from low to high level.

The CPU 24 starts to control the exposure of the shutter mechanism andthe timing at which the strobe is synchronized when the output from thecomparator 21 changes into high level. After one photography operationis completed, the control signal S2 goes high, turning on the transistor25. In this way, the electric charge stored in the capacitor 22 isreleased.

FIG. 3 shows an example in which the present invention is implemented bysoftware instead of the analog circuit described above. FIG. 4 is aflowchart illustrating it. Like components are indicated by likereference numerals in various figures including FIG. 2 and thosecomponents which have been already described will not be describedbelow. A terminal voltage VL is developed across the resistor 20 by aphotocurrent flowing through the phototransistor 15b. This terminalvoltage VL is converted into digital form by an analog-to-digitalconverter circuit 29. The converted value is applied to a CPU 31 via aninterface 30.

The operation is described now in detail. The driving pulses aresupplied to the stepper motor (not shown). The light-emitting diode 15ais made to emit previously. The terminal voltage VL developed across theresistor 20 at this time is converted into digital form and sampled. Theresulting value is multiplied by approximately 1/2. The product isstored as a reference voltage Vref. Then, the opening lever 14 isreleased to cause the shutter blade 10 to open the aperture. Thereafter,the terminal voltage VL across the resistor 20 is monitored. When theterminal voltage VL exceeds the reference voltage Vref, the shutterblade is regarded as having reached a desired position. The followingexposure sequence is executed.

In the above examples, a photo-reflector is used as one example of thesensor means, and the present invention is applied to detection of theposition of the shutter blade. The invention can be applied to othermeans. Of course, a light switch such as a transmission-typephoto-interrupter can be used. The invention can be applied to varioussensor means other than optical switches as long as the analog outputfrom the sensor means varies, depending on whether the detecting portionis operating or not. Furthermore, no problem takes place if the detectedobject can be other than a shutter blade. For example, FIG. 5 shows anexample in which a decision is made as to whether a toothed wheel 33such as a motor pinion inside a camera has reached a given angularposition, using a transmission-type photo-interrupter 32. The presentinvention can be directly applied to this case.

INDUSTRIAL APPLICABILITY

As described thus far, according to the present invention, prior to anoperation for detecting a position in practice, the output level from adetecting means is preliminarily read. The threshold level of acomparator means is established according to the level of the outputfrom the sensor means during the preliminary reading. Therefore, it isnot necessary to adjust the operating point according to thecharacteristics of the used sensor means or to install an accuratevoltage-regulated power circuit. Also, if the sensor means ages anddeteriorates in performance, the threshold level can be automaticallycorrected each time a detection operation is performed. Hence, thedetecting operation can be carried out without being affected by aging.In addition, where the load conditions during previous emission are madecoincident with the load conditions during detection as in the aboveexamples, the effects of drop of the power level caused by loadvariations can be substantially avoided.

I claim:
 1. A position sensor comprising:sensor means for detecting aposition of an object, said sensor means having a detecting portiondisposed along an operation line of said object, said sensor meansproducing an output signal; threshold level determining means forreading a first output level of said output signal from said sensormeans to determine a threshold level from said first output level; andcomparator means for comparing a second output level of said outputsignal from said sensor means with said threshold level and forproducing a position signal indicative of said position, said thresholdlevel determining means reading said first level of said output signalprior to said comparator means producing said position signal toestablish said threshold level of said comparator means according tosaid first level of said output signal.
 2. The position sensor of claim1, wherein said sensor means includes light-emitting means for emittinglight and light-receiving means for receiving said emitted light, saidposition sensor including an optical path going from said light-emittingmeans to said light-receiving means to form a light switch that isoperated by said object to be detected.
 3. The position sensor of claim1, wherein said comparator means includes means for producing saidposition signal when said second output level from said sensor meansexceeds said threshold level.
 4. The position sensor of claim 3, whereinsaid threshold level is determined based on one-half of said firstoutput level obtained from said sensor means.
 5. A position sensorcomprising:sensor means for sensing a position of an object, said sensormeans having a detecting portion disposed along an operation line ofsaid object to be detected, said detecting portion producing an analogoutput signal which varies based on an operating state of said detectingportion; and comparator means for producing an output signal when saidanalog output signal from said sensor means changes from a first levelindicating non-operation of said detecting portion to a second levelindicating operation of said detecting portion across a threshold level,said object having a portion for permitting said detecting portion toshift from a first operating state to a non-operating state and then toa second operating state during detection of said position of saidobject, said threshold level of said comparator being establishedaccording to said analog output signal from said sensor means when saiddetecting means is in said first operating state.
 6. The position sensorof claim 5, wherein said object is operated when said detecting portionis in said first operating state.
 7. The position sensor of claim 5,wherein said sensor means is equipped with a light-emitting means foremitting light and a light-receiving means for receiving said emittedlight, said position sensor including an optical path going from saidlight-emitting means to said light-receiving means to form a lightswitch that is operated by said object.
 8. A position sensorcomprising:sensor means for sensing an object, said sensor means havinga detecting portion disposed along an operation line of a shutter bladeto block and unblock an optical path going from a subject to an imageplane, said sensor means producing an analog output signal which variesin accordance with an operation of said detecting portion; andcomparator means for producing an output signal when said analog outputsignal from said sensor means changes from a first level indicatingnon-operation of said detecting portion to a second level indicatingoperation of said detecting portion across a threshold level, saidshutter blade including shifting means for shifting said detectingportion from a first operating state to a non-operating state and thento a second operating state during operation of said shutter blade, saidthreshold level of said comparator means being established according tosaid analog output signal from said sensor means when said detectingportion is in said first operating state.
 9. The position sensor ofclaim 8, wherein said shutter blade is operated when said detectingportion is in said first operating state.
 10. The position sensor ofclaim 8, wherein said sensor means includes light-emitting means foremitting light and a light-receiving means for receiving light, saidsensor means also including an optical path going from saidlight-emitting means to said light-receiving means to form a lightswitch that is operated by said shutter blade.